As described in the prior art, there are several methods for the determination of organolithium compounds.
Kofron and Baclawski [Kofron, W. G.; Baclawski, L. M.; J. Org. Chem. 1976, 41, 1879] describe the use of diphenyl acetic acid as colorimetric indicator.
As alternatives to diphenyl acetic acid, similar indicators have been proposed. 4-biphenylyl methanol, 4-biphenylyl acetic acid [Juaristi, E.; Martínez-Richa, A.; García Rivera, A.; Cruz-Sánchez, J. S.; J.Org. Chem. 1983, 48, 2603], and 1-pyreneacetic acid [Kiljumen, H.; Hase, T. A.; J.Org.Chem. 1991, 56, 6950] have been used as indicator molecules for the titration of organolithium compounds providing a clearer visual detection of the equivalence point due to a more extended delocalization of the charge in the coloured final dianions.
In the same way, Winkle et al. [Winkle, M. R.; Lansinger, J. M.; Donald, R. C.; J. Chem. Soc. Chem. Commun. 1980, 87] have used 2,5-dimethoxybenzyl alcohol as indicator for the titration of organolithium reagents, resulting in the formation of visually detectable coloured dianionic species.
More recently, new titration methods for the determination of alkyl- and aryllithium compounds based on the double deprotonation of different colorimetric indicators have been reported. This is the case of the procedure described by Suffert [Suffert, J.; J.Org.Chem. 1989, 54, 509] proposing N-pivaloyl-o-toluidine and/or N-pivaloyl-o-benzyl aniline as indicator dyes.
Another group of indicators that undergo a double deprotonation upon reacting with lithium organometallic compounds include benzenesulphonyl- and tosylhydrazone derivatives [Miranda, R.; Hernández, A.; Angeles, E.; Cabrera, A.; Salmón, M.; Joseph-Nathan, P.; Analyst, 1990, 115, 1483].
Another possibility of determining organolithium compounds by colorimetric titration is based on the single deprotonation of triphenylmethane [Eppley, R. L.; Dixon, J. A.; J. Organomet. Chem 1967, 8, 176], N-benzyliden-benzylamine [Duhamel, L.; Plaqevant, J. C.; J.Org.Chem. 1981, 44, 24304] or N-phenyl-1-naphthylamine [Bergbreiter, D. E.; Pendergrass, E.; J.Org.Chem. 1981, 46, 219] to yield, after reaction with the organolithium compound, the corresponding coloured anions that can be back-titrated with sec-butyl alcohol or benzoic acid solution.
Moreover, the fact that alkyl organolithium and organomagnesium compounds form coloured complexes with different Lewis bases, all of them having extended π or aromatic structure, provides us one of the more used optical methods for the titration of such organometallic compounds. The charge-transfer complexes obtained in this way display an intense absorption in the visible region of the electromagnetic spectrum. The most common polycyclic aromatic bases used to that end are 1,10-phenanthroline and 2,2-biquinoline [Watson, S. C.; Eastham, J. F.; J.Organomet.Chem. 1967, 9, 165].
On the other hand, industrial anionic polymerization initiated by organolithium compounds is one of the more versatile methods for synthesizing macromolecules due to the possibility of designing very different chemical structures for many applications, but an strict control of the process is required. Living anionic polymerization of 1,3-conjugated diene and/or monoalkenyl aromatic hydrocarbon, allows us to synthesize macromolecular compounds with a high control of their composition and structural parameters such as molecular weight distribution, microstructure, stereochemistry, branching and chain functionalization. The molecular weight of the polymer is one of the most important variables that affect the macromolecular properties. In a living anionic polymerization, the molecular weight is determined by the reaction stoichiometry and the conversion rate. This solution polymerization process is extremely sensitive to impurities in the reaction medium that can modify the effective initiator concentration or the number of growing chains. That is the reason why one of the main parameters to be controlled in such processes is the so called reaction threshold, namely, the amount of initiator consumed by impurities or “poisons” able to react with it before the initiation step is effective.
Therefore, according to the background, a method based on sensors useful for titration of organolithium compounds by spectroscopic monitorization of reactive species has not been described up to date, with particular application to anionic polymerization processes initiated by organolithium compounds or any other process that involves an organometallic compound being critical the stoichiometry of this compound.